A number of present day wireless data networks transmit data between their base stations and mobile units in a series of fixed-length, physical layer blocks (hereafter simply referred to as a "block". Each block comprises a number of payload bits and parity bits, which are generated by a forward error correction code or coding scheme. In general adding more parity bits per block increases the number of airlink errors that can be detected and corrected.
However, having a large number of parity bits per block has an obvious downside--it decreases the number of available payload bits. As a result, wireless data networks such as General Packet Radio Service (GPRS) networks use more than one coding scheme to transmit data over the airlink. When the received signal-to-noise ratio (SNR) is high, airlink bit error rates are low. As a result, a coding scheme with a small number of parity bits may offer adequate protection. At low SNR, "stronger" codes may be needed to protect data against airlink errors since stronger codes add more parity bits to each block.
The error performance of a cellular airlink varies as mobile units move within a cell. To make most efficient use of the airlink, coding schemes are dynamically selected in response to changes in the quality of the cellular airlink. Current coding scheme selection algorithms are a function of a channel quality metric (CQM). A CQM is, e.g., a function of soft bit or soft symbol information, block or bit error rate estimates, received signal strength, and/or the carrier-to-interference ratio (C/I).
For example, with respect to C/I, for a given coding scheme, the portion of transmissions which result in block errors decreases as the C/I value of a received signal increases. Using simulation or analytical techniques, it is possible to estimate the rate at which payload bits are carried over the airlink as a function of C/f. Plots of throughput-versus-C/I curves for all coding schemes available in a wireless data network show at which values of C/I it is advantageous to switch coding schemes. An illustrative throughput-versus-C/I plot is shown in FIG. 1 for three coding schemes I, II, and III, where coding scheme I is the strongest and coding scheme III is the weakest. C/I switch points are often hard-coded at the transmitter. Based on C/I measurements, a transmitter switches to a coding scheme offering the best performance (highest throughput/lowest delays) at the estimated received C/I level.
Similar selection techniques are used in other systems, e.g. Enhanced GPRS and North American TDMA Packet Data Channel. For example, instead of, or in addition to, the channel coding rate, the modulation scheme (signal constellation size) is varied to achieve a similar tradeoff for variable C/I. Thus, in the North American TDMA Packet Data Channel, the channel coding rate is fixed (at 5/6), while the modulation scheme is switched between 4-level (DQPSK) and 8-level (coherent 8 PSK) with a possible extension to 16-level (not yet specified). In this case, schematically the same performance tradeoffs as shown in FIG. 1 apply, if we now refer to the three formats I, II, and III as three modulation schemes. (As such, as used herein, the term "coding/modulation scheme" refers to either a coding scheme, modulation scheme, or coding and modulation scheme used to transmit a signal.)